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SLATER, ROBERT L., DATUS M. HAMtVOND, AND MERTHYB L. MINER. 1970. Eimeria boris: Development in Calves Treated with Thiamine Metabolic Antagonist (Amprolium) in Feed. Trans. Amer. Microsc. Soc. 89: 55-65. Fifty-seven calves about 2 to 3 months old were used in four experiments, in each of which each calf was given 50,000 Eimeria bovis oocysts by means of a nippled flask. In three experiments, 2 groups each of 3 or 4 calves were treated with 25 and 5 mg/kg of a thiamine metabolic antagonist (amprolium) respectively, and one group was not treated. In the fourth experiment, an additional group of calves was treated with 1 mg/kg of amprolium. The amprolium was given once daily in the morning grain feeding for 21 days beginning on the day before inoculation. In three of the experiments, 1 or 2 calves in each group were killed 14 to 19 days after inoculation to determine the effect of ainprolium upon the endogenous stages of E. bovis. In the calves treated at 25 mg/kg and at 5 mg/kg, little or no clinical signs of coccidiosis were observed and only a few or no oocysts were discharged. The oocyst discharge occurred 4 to 7 days later than normal in the calves treated at 25 mg/kg. Treatment at 1 mg/kg was not effective. Coccidia were found in the tissues in calves killed 14 to 19 days after inoculation. Sixty-four of 81 schizonts found in calves treated at 25 mg/kg appeared to be retarded in development. Only 7 of 30 schizonts seen in the calves treated at 5 mg/kg and killed 14 to 19 days after inoculation showed such a retardation. These retarded schizonts were about the size and stage of development normally occurring in calves killed 8 to 10 days after inoculation. In the calves treated at 25 mg/kg, most of the host cells harboring retarded schizonts were excessively hypertrophied and some showed vacuolization. Evidence of degeneration of the schizonts was seen in these treated calves, especially those killed 19 (lays after inoculation. Under the conditions of these experiments, amproliium evidently affected the first-generation schizonts, causing many of theml to be retarded in development or to fail to develop beyond the early stages.